The indications for a spondylodesis (stiffening) of the entire spinal column or adjacent vertebrae may be very complex. Stabilizations, for example, may be performed in the case of degenerative change in the spinal column, after tumor removal, infections, or trauma.
In some cases, combinations of dorsal systems (e.g. via rod-pedicle-screw systems) combined with multiple cages for ventral bracing may be used for stabilizing the spinal column.
The statics of the spinal column, which may be a weight-bearing unit, can be compared with those of a crane. The anterior column, which may consist of vertebrae and intervertebral discs, may bear about 80% of the weight and the dorsal structures may bear approximately 20%. From this we can deduct that the anterior portion of the lumbar spine may be primarily subject to compressive forces, while the dorsal portion may be primarily subject to tractive forces. In addition, shearing, torsional, and bending forces may act on both columns.
While the above-mentioned screws/rod-systems may be used in the dorsal area, a cage or implant may be used for ventral bracing.
An objective of the stabilization of the vertebrae may be a quick ossification of the intervertebral space, which may permit lasting freedom from and/or reduction in pain for a patient.
Examples of the indication of a spondylodesis may include unstable vertebral fractures, degenerative instabilities, fractures with sufficient anterior bracing, dislocations, spinal tumors (without anterior defects), and/or prior failed fusion (pseudoarthrosis).
The surgical techniques may be carried out in an open fashion or as minimally invasive procedures, depending on the indication. These procedures may differ in size and type of access or accesses.
Accesses to the affected area can be gained through a combination of a ventral and dorsal access (in case of the cervical spine sometimes ventral only) or through a dorsal, dorsolateral, or lateral access.
An implant of the appropriate dimensions may be chosen depending on the structures and proportions found. Generally, the size may be calculated in such a fashion that the screw reaches into the anterior third of the vertebral body.
Pedicle screws may be screwed into the vertebral body through the pedicle. For safe navigation and guidance, some screws may be cannulated and may be inserted via a guide wire. Some pedicle screws may include additional cross-holes in the thread piece for subsequent cementing.
A rod, which may connect two or more pedicle screw with each other, may be inserted into the head of the screw, which may be called a “tulip”. The tulip may be rigid or movable in one or multiple directions, which may facilitate later insertion of the rod.
A rod may be inserted on both sides, and the vertebral bodies to be fixed may then be pressed apart with varying spreading systems, generally via the pedicle screws, to achieve an optimal distance between the vertebral bodies.
During this process, the mobility of the tulip may be obstructive if the objective is to raise vertebral bodies or reconstruct a physiological alignment (e.g., in accident surgery).
Minimally invasive procedures may use an extension of the tulip, which may allow for the extracorporeal, percutaneous insertion of the rod.
Depending on the surgical technique, a variety of a manufacturer's screw forms and associated instrument sets may be used.
It may be disadvantageous to use a variety of different instruments as it may then be necessary for surgeons and surgical assistants to learn and/or remember the specifics of each instrument and screw combination and/or it may be necessary to rely on the manufacturing companies' product managers.